This invention relates in general to thermoplastic polymers for structural applications and, more specifically, to a method of improving the physical properties of such polymers through crosslinking.
Fiber reinforced synthetic resins have come into increasing use in structural applications. Glass fibers embedded in thermosetting resins have long been used for automobile bodies, sporting goods, etc. Recently, composites comprising graphite or boron fibers embedded in epoxy or polyester resins have been used in applications such as aircraft components, golf club shafts, etc. where a very high strength to weight ratio is desirable.
The thermosetting matrices have several problems limiting their usefulness. Once set, the matrix cannot be reshaped. The resin must ordinarily be applied as a liquid catalyst resin in a hard lay up mode, requiring complex molds and highly skilled operators. Some resins, such as the epoxys, may be applied to the fibers and cured to a tacky "B-stage". Such preimpregnated or "pre-preg" materials have limited shelf lives and are tacky and difficult to assemble on a mold for heating under pressure to produce a final structure component. Thus, thermosetting-matrix composites must ordinarily be fabricated in a controlled, factory environment.
The need for reshaping composite materials or construction of structural shapes in the field have led to the increasing use of composites in which the high strength fibers are embedded in a thermoplastic resin matrix. Such materials are in a dry, easily handled state and can be heated to the softening temperature, reshaped and cooled to produce desired product shapes. This is especially useful where beams, panels, etc. are to be formed from coils of sheet composite material by automatic machining, such as might be used in space to automatically fabricate structural components in building a space station. Unfortunately, thermoplastic materials tend to be susceptible to solvents, have a low thermomechanical limit and creep under load. Also, some thermoplastics even when heated to a softening temperature are stiff and difficult to reshape in Yoder rolls, etc.
Thus, there is a continuing need for methods of producing resin matrix materials for fiber-reinforced composites which have the strength and environmental damage resistance of thermosetting resins and the ease of forming and reshaping of thermoplastics.